Chemical Engineering June 2020 - 24

Facts At Your Fingertips
Water Treatment: Ion Exchange Resins
Department Editor: Scott Jenkins
emoval of dissolved ions from
industrial process water, boiler
feedwater or wastewater is a
common challenge for facilities in the
chemical process industries (CPI).
Ion exchange (IX) systems offer one
possible solution for demineralization,
purification or wastewater-treatment
applications. Ion exchange
systems facilitate reversible chemical
reactions in which the ions targeted
for removal are captured electrostatically
and replaced by other ions of
similar charge. Key components of
IX systems are ion exchange resins.
This one-page reference provides an
overview of how IX resins work in industrial
water treatment.
R
Resin characteristics
IX resins are generally composed
of crosslinked organic polymers
shaped into small beads usually
measuring between 0.25 and 1.25
mm in diameter. The most common
IX resin material is polystyrene with
divinylbenzene crosslinks. This type
of resin is used in the majority of IX
applications. Although the basic resin
components are often the same,
the resins have been modified in
many ways to meet the requirements
of specific applications and provide
longer resin life.
The polymer matrix of the beads
contains dispersed ion-active sites,
which are charged functional groups
covalently bound to the polymer
backbone. The charged functional
groups, which can be negative or
positive, attract ions of the opposite
charge through electrostatic interactions.
For example, a common
functional group found in cation IX
resins is the sulfonate anion (SO3-).
A counterion solution containing sodium
cations (Na+) is applied to the
resin. The Na+ ions are held to the
bound SO3- anions by electrostatic
attraction, so the resin carries a net
neutral charge (Figure 1).
Counterions for the resin are chosen
such that when they are exchanged
and are released into the
treated water, they will not present
problems for subsequent use,
24
and the problematic ions will
be removed. In general, the
greater the size and valency
of an ion, the greater affinity it
will have with ions of an opposite
charge.
IX operations
In general, water containing
undesirable ions flows
through a bed of resin beads
and the problematic ions are
exchanged for ions of similar
charge that were first applied
to the resin. The functional
groups and initial counterions
are designed such that the ions
in the treated water will have a greater
affinity for the charged functional
groups and will dislodge the existing
ions and take their place, bonding
with the functional groups through
shared electrostatic attraction [2].
For example, in water-softening
applications, water containing Ca2+
or Mg2+ ions flows through the resin
bed. Since the SO3-
functional
groups have a greater affinity for the
hardness cations than for the Na+
ions, the hardness ions displace the
Na+ ions, which then flow out of the
IX unit as part of the treated stream,
while the Ca2+ or Mg2+ are retained
by the IX resin [2].
The impurities are captured from
the feedwater until they saturate the
IX resin material, after which they
must be flushed out with a regenerant
solution. These typically consist
of concentrated salts, acids or bases.
By flushing the resin, the contaminant
ions are released into a waste stream,
and the cations and anions in the resin
are restored for another cycle.
Resins are designed to resist thermal
degradation and oxidizing agents,
as well as organic fouling. In addition
to polystyrene-divinylbenzene resins,
there are newer resins with an acrylic
structure, which increases their resistance
to organic fouling.
Resin categories
The ionizable groups attached to the
resin bead determine the functional
capability of the resin. Generally, inPolystyrene
backbone
H3C
CH3
n
Electrostatic
interaction
Bound
functional
group
O
O
S
O-...Na+
Figure
1. Charged funtional groups covalently bound to the
polymer backbone of the ion exchange resin are key to its
ability to remove undesirable ions from treated water
dustrial-water-treatment resins are
classified into four categories:
Strong acid cation (SAC) exchange
resins. SAC resins are composed of
a polystyrene matrix with a sulfonate
(SO3-) functional group that is either
charged with sodium ions for water
softening applications, or hydrogen
ions (H+) for demineralization.
Weak acid cation (WAC) exchange
resins. WAC resins are composed
of an acrylic polymer with carboxylic
acid functional groups. Due to their
high affinity for hydrogen ions (H+),
WAC resins are typically used to selectively
remove cations associated
with alkalinity.
Strong base anion (SBA) exchange
resins. SBA resins are typically composed
of a polystyrene matrix with
fixed anions at exchange sites that
yield either chloride ions or hydroxide
ions (OH-).
Weak base anion (WBA) exchange
resins. WBA resins are unique in that
they do not have exchangeable ions,
and are therefore used as acid absorbers
to remove anions associated
with strong mineral acids.
Chelating resins are also available
for selective removal of certain metals
and other substances.
n
References
1. Suez Water Technologies & Solutions, " The Handbook of Industrial
Water Treatment, " Chapter 8, online publication, accessed
April 2020 at www.suezwatertechnologies.com.
2. SAMCO Technologies, " Understanding the Basics of Ion
Exchange Systems, " SAMCO, e-book, 2017.
3. Dupont Water Technologies, Ion Exchange Resins Tech Fact,
Dupont, 2019.
ChemiCal engineering www.Chemengonline.Com June 2020
http://www.suezwatertechnologies.com http://www.Chemengonline.Com
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